The requirements for output power control in mobile telephones can often be difficult to achieve. The requirements for GSM, for example, can be found in the ETSI specification “05.05 Digital cellular telecommunications system; Radio transmission and reception”. There are three critical parameters concerning the transmitter output power.                Output power level during a constant power part (“mid part”) of a transmitted burst.        Power vs. time, i.e. output power during up-ramping and down-ramping parts of a transmitted burst.        Spectrum due to switching (up- and down-ramping).        
Several output power classes are specified in the 05.05 document. These power levels should be kept within well-defined tolerances.
The “power vs. time” requirements state that the transmitted power should fit within a specified template, of output power versus time. The template can be illustrated as a graph of power vs. time. Adjusting the telephone parameters so that they fit the power vs. time template can be a very time-consuming task during development and critical during manufacturing.
The spectrum due to switching requirement means that the spectrum caused by the ramping (switching) process should fit in a specified spectrum mask. It is therefore necessary to have a “good” (reliable) power vs. time behaviour, not only to fulfil the power vs. time template but also to avoid spectrum contamination.
It is to be noted that, although the GSM system is used as an example, the ideas presented in this specification could be used in any TDMA (Time Division Multiple Access) system, or any system that requires fast and/or accurate power control, such as CDMA.
Arranging the power control so that the telephone fits the power vs. time template and the spectrum due to switching mask, can be a very time-consuming task. In production, good yield is necessary.
In FIG. 1 of the accompanying drawings, the principle of today's power control solution is shown. A power amplifier 1 is connected to receive an RF input RFin. The power amplifier operates to output an amplified RF signal RFout to an antenna 2, as is known and understood.
In order to control the power output of the power amplifier, the current, Ic, used by the PA (Power Amplifier) 1, is measured (through a resistor R). This current value provides an indirect measurement of the PA output power. The measurement provided by voltage x4=RIc, is fed back for comparison with an input control signal x1 in an error detection unit 3. A difference (or error signal), x2, is filtered by a loop filter, HLP, to produce a control signal x3, which is used for controlling the PA RF output power. Signal x3 is often called Vapc (apc=amplifier power control). Ideally, the measurement signal x4 should track the input control signal x1.
The total transfer function for the control system (Htot=x4/x1) can be found from the following:x4=x3HPA=x2HLPHPA=(x1−x4)HLPHPA  (1)x4(1+HLPHPA)=x1HLPHPA  (2)
                              H          tot                =                                            x              4                                      x              1                                =                                                    H                LP                            ⁢                              H                PA                                                    1              +                                                H                  LP                                ⁢                                  H                  PA                                                                                        (        3        )            
Minimizing the difference between x1 and x4 would provide an ideal control loop. This means that x4/x1≈1, or HLPHPA>>1.
Ideally, the transfer function HPA=x4/x3 should be constant(=Ic/Vapc). However, in practice, this is not generally the case. As illustrated in FIG. 2, the transfer function of the feedback loop typically varies, i.e. the feedback loop gain varies. This variation is due to the variation of the PA transfer function HPA with the control voltage Vapc. Thus, the maximum achievable error reduction of the control system will vary. In the FIG. 2 example, the loop is practically “open” for low Vapc and high Vapc values, causing poor tracking ability in the control system. For medium Vapc values however, the tracking ability is good, since the loop gain is high.
The non-constant behaviour of HPA will also result in implementation difficulties for the loop filter since the risk of instability is high. The reason for this is that the loop filter must have sufficient gain to achieve good error reduction and fast control even at low or high Vapc values (where HPA is small). On the other hand, this means increased risk for instability at medium Vapc values (where HPA is large).